Protective coating

ABSTRACT

THIS INVENTION RELATES TO A METHOD FOR PROTECTNG AND TO A PROTECTIVE COATING FOR USE ON HOT METAL WORKPIECES, PARTICULARLY HIGH-SPEED TOOL STEELS, TO PROTECT THE SURFACE AGAINST SCALING AND DECARBURIZATION DURING ELEVATED-TEMPERATURE PROCESSING FROM INGOT TO BILLET. THE COATING CONSISTS OF SODIUM BORATE, SILICA, AND A VISCOSITY-PROMOTING AGENT, SUCH AS ALUMINA.

Oct. 17, 1972 w, R CQLANTUQNO 3,698,943

PROTECTIVE CQATING 2 Sheets-Sheet 1 Original Filed May 28. 1968 O O 0 "O0 0 Qt 3t MQQMQNIQY MS 2t 3t MEFMQNIQQ IN VEN TOR.

WILLIAM R. COL UONO A I farney Oct. 11, 1972 R. COLANTUONO 3,698,943

PROTECTIVE COATING 2 Sheets-Sheet 2 I Original Filed May 28, 1968 more-c TION-RA TING vs PERCENT 5,0

3 2 2t St 2Q: .umkbtl Adherence flaring PROTECT/0N RAT/N6 0F VERTICAL SIDES V8 PERCENT 4/ 0 w 0 y T e N n w w A r o 6 m d M /WW o fi 1 o l M 5 L 6 M o 9 B o I O a o I A o T o w R E P 0 2 O a -1 3 n O United States Patent 3,698,943 PROTECTIVE COATING William R. Colantuono, Pittsburgh, Pa., assignor to Crucible Inc.

Original application May 28, 1968, Ser. No. 732,612. Divided and this application Sept. 3, 1970, Ser. No. 69,439

Int. Cl. B44d 1/34; C23f 1/44 US. Cl. 117-53 3 Claims ABSTRACT OF THE DISCLOSURE This case is a divisional of Ser. No. 732,612 filed May 28, 1968.

In the processing of high-speed tool steels, and particularly molybdenum-base steels of this type, it is customary to subject the steel ingot to elevated-temperature process ing, which includes hot-working, from an ingot to an intermediate billet product. At the high temperatures characteristic of this processing, the workpiece is subject to atmospheres that produce scaling the decarburization. The scaling, resulting from oxidation, causes a substantial loss of product yield, and decarburization necessitates surface conditioning of the workpiece, the magnitude of which depends upon the decarburization depth.

It is customary to prevent both scaling and decarburization during high-temperature processing of metal workpieces, other than ingots, by the application of various protective coatings, which remain on the surface of the workpiece during the processing sequence. Customarily, these coatings, which contain high-temperature melting constituents, are applied to a cold workpiece which is subsequently heated to the elevated temperatures required for processing. In billet production from tool steel ingots, however, the ingot usually is not permitted prior to high-temperature processing to come to room temperature. Consequently, any protective coating must be applied, prior to high-temperature processing, to an ingot that is within an intermediate temperature range of about 1000 to 1400 F. At this temperature, conventional coatings customarily used for protection against scaling and decarburization will not adhere to the hot ingot surface, because at a temperature of about 1000 to 1400 F., the high-melting constituents of the coating are not heated to their melting temperature. Hence, before reaching the extremely high temperatures required in processing to highspeed tool steel billets, which may be on the order of 2000 F. or higher, conventional coatings lack the required adherence. Therefore, they are removed from the workpiece surfaces to be protected prior to or early in the processing incident to tool-steel billet production.

It is accordingly the primary object of the present invention to provide a method for protecting high-speed tool steel workpieces against high temperature oxidation and decarburization during high temperature processing.

Another object of the invention is to provide a protective coating, particularly suited for use on high-speed tool steel workpieces, that gives the required protection against scaling and decarburization at the high temperatures incident to billet production, and yet will adhere to the workpiece at the substantially lower temperatures of the ingots at the time the coating is applied.

Another object of the invention is to provide a protective coating for high-speed tool steel workpieces that will adhere to an ingot at a temperature on the order of 1000 F. and protect the workpiece from oxidation and decarburization during the long, high-temperature heating cycles to temperatures above 2000 F. incident to the hot working of the tool steel ingot to a billet product.

Another more particular object of the invention is to provide a protective coating for use on highspeed tool steel ingots to protect the same against oxidation and decarburization during processing to billets that will not impair the hot-working operations due to any reduction in the friction between the workroll surfaces and the workpiece surfaces.

These and other objects of the invention, as well as the complete understanding thereof, will be evident from the following description, specific examples and drawings, in which: I

FIG. 1 is a graphic presentation of data relating to the adherence properties of the coating of the present invention;

FIG. 2 is a graphic presentation of data relating to the properties of the coating in protecting tool steel workpieces against high temperature oxidation and carburization; and

FIG. 3 is a graphic presentation of data relating to the amount of alumina required in the coating to provide the viscosity necessary for the purpose of the invention.

In the practice of the invention the protective coating In the practice of the invention the protective coaing comprises an admixture of sodium borate (Na B O- silica (SiO and alumina (A1 0 Silica is present in the coating within the range of 10 to 55 percent by weight and preferably Within the range of 25 to 45 percent by weight. The total coating constituents are limited to provide an adherence ratio, which term will be defined hereinafter, of at least 0.2 and preferably at least 0.3. The protection rating and the adherence rating of the coating, which are defined hereinafter, must be at least 3 for the purpose of achieving the objects of the invention. Boric acid in anhydrous powder form may be substituted for a minor portion of the sodium borate in the coating material.

The sodium borate in the coating, which may be regarded as a low-temperature melting constituent and a [flux that lowers the melting point of the other constituents of the coating, functions in the coating to adhere the coating to the workpiece, such as a tool steel ingot, at an intermediate temperature on the order of for example 1000 F. The silica and the alumina, which remain unmelted at these relatively low ingot temperatures, are consequently bonded to the ingot at such temperatures by the action of the sodium borate. In applications wherein the coating material is applied to ingots at lower-thannormal temperatures, boric acid anhydrous powder may be substituted for a minor portion of the sodium borate; the substitution of the boric acid, which at a low melting temperature, e.g. about 700 F., will serve to lower the melting temperature of the low-temperature melting constituents and thus enable it to adhere to workpieces, such as ingots, at relatively lower temperatures, e.g., less than for example 1000 F.

During high-temperature processing, which includes heating the workpiece or ingot to temperatures of 2000 F. or higher for hot-working, the previously unmelted silica and alumina fluxed by the sodium borate will melt to provide a continuous coating over the workpiece surface to be protected. The desired protection against oxidation and decarburization is provided by the silica of the coating material, which at the high temperatures incident to billet production forms an adherent, dense continuous surface layer over the metal workpiece. This surface layer prevents exposure of the metal surface to the oxidizing atmosphere and thereby prevents surface oxidation or scaling. In addition, decarburization of the workpiece surface is prevented by shielding the surface against any carbon reaction with the surrounding oxygen-containing atmosphere.

The aluminum content of the coating serves a dual function. First, it acts as a viscosity-promoting agent to reduce the fluidity of the coating and thus prevents it from running off the vertical surfaces of the workpiece during processing. Also, the alumina serves to increase the friction between the coated workpiece surface and the workrolls used in reducing the ingot to billet form during hot-working. In the absence of alumina, there is not sufiicient friction between the coated workpiece surface and the workrolls and, consequently, slippage occurs during rolling, which detracts from the efficiency of the rolling operation.

It may be seen from the above, therefore, that the coating of the invention is particularly adapted for use in the production of tool steel billets in that it adheres to the ingot at intermediate temperatures of about 1000 F. when initially applied and subsequently protects the ingot from oxidation and decarburization during the long heating cycles to hot-working temperatures of 2000" F. and higher, while not inhibiting the hot-working of the material by reducing the friction between the workpiece surfaces and the workroll surfaces. It is to be understood, of course, that upon completion of processing from a tool steel ingot to billet form, the coating of the invention is readily removed by the metal flow incident to hotworking. The coating may, if desired, be formed in situ by first applying a quantity of sodium borate substantially continuous over the workpiece surface to be protected. After melting of the sodium borate, the silica and alumina may then be deposited thereover. The fused sodium borate will serve to bond the alumina and silica to the workpiece surfaces at the intermediate temperature. It is the preferred practice, however, to provide the sodium borate, silica and alumina in admixture and apply the admixture to the surface of the ingot while at typical intermediate temperature.

It should be recognized that for the coating of the invention to properly function in its intended application for preventing oxidation and decarburization of tool steel ingots during high-temperature processing to billet form, it must adhere to the ingot surface at the relatively low, intermediate, temperature and provide the required protection against the ambient oxygen-containing atmosphere during the high-temperature processing cycle incident to hot-working. Obviously, the coating material coating does not adhere at the low temperatures so that areas of the workpiece surface are exposed to the oxidizing atmosphere during high-temperature processing, then no matter what the otherwise protective properties of the coating may be, the exposed areas of the workpiece will be subjected to oxidation and decarburization when processed at elevated temperature. Conversely, the coating will not be useful if it provides complete adherence and yet does not serve to protect the workpiece surface against the high-temperature oxidizing atmosphere during processing to billet form.

The adherence of the coating, in accordance with the present invention, has been found to be governed by the total surface area of the sodium borate (low-temperature melting constituent), which will melt at the application temperature, divided by the total surface area of the silica and alumina (high-temperature melting constituents), which will remain unmelted at the application temperature. This function, which may be termed the adherence ratio, may be expressed as follows:

Total surface area of sodium borate Total surface area of silica plus total surface area of alumina It has been found, as will be demonstrated by the specific examples reported hereinafter, that this adherence ratio, for the purpose of providing adequate adherence at an intermediate elevated temperature, must be at least 0.2 and preferably at least 0.3. It should be noted that an additional factor affecting the total surface area of the sodium borate and thus the adherence ratio is the mesh size of the coating constituents. By decreasing the mesh size of the sodium borate, the adherence ratio will be accordingly increased. Conversely, by increasing the size of the silica and alumina, the adherence ratio will likewise be increased. Both of these factors will, of course, serve to increase the total surface area of the sodium borate in relation to the silica and alumina which function, as explained hereinabove, determines the adherence ratio.

For the purpose of providing protection of the workpiece against oxidation and decarburization by providing a continuous barrier at high temperature against the surrounding oxygen-containing atmosphere, silica must be present within the range of 10 to 55 percent by weight and preferably within the range of 25 to percent by weight. T 0 provide a coating with viscosity sufficient to prevent it from running off the vertical surfaces of the workpiece during processing, alumina should be present in an amount of at least about 25 percent by weight. An upper limit for alumina would be about 60' percent by weight. It is understood that it is necessary in combination with this silica limitation to have an adequate adherence ratio so that the coating remains continuous over must serve both of these functions to be useful. If the the workpiece surface to be protected.

TABLE I Parts weight of- Percent Adher- Protec- Adher- Vertical ence tion ence rotection Sample Na2B4O S101 A1 0 S102 A120 ratio rating rating p rating Borax 1 (-+80) 0 0.0 0.0 1 K 1 (60+80) 0 2 (-48-l-6 0. 0 66. 6 1. 16 1 L 1 (60+80) 1 (60+80) 2 (48+60) 25. 0 50. 0 0.54 3. U. l (60+80) 1. 5 (60+80) 3. 5 (48+60) 25.0 58. 5 O. 32 3. 5 P- 1 (60+80) 1. 5 (60+80) 2 (48+60) 33. 3 44. 5 0. 42 3. 5 V 2 (60+80) 2 (48+60) 40. 0 40. 0 0. 49 4. 0 S. 2 (60+80) 2 (60+80) 40. 0 40. 0 0. 265 1. 5 R 2 (--l-120) 2 (60+8 40. 0 40. 0 0. 17 1. 5 W 2 (--60+80) 2 (-48-1-60) 40. 0 40. 0 0. 45 3. 75 T 2. 5 (60+80) 2. 5 (-48-1-60) 41. 7 41. 7 0. 2 2. 75 Q 2. 5 (60+8 2 (-484-60) 45. 5 36. 3 0. 37 4. 0 M 3 (-60-l-80) 2 (-48-Hl0) 50. 0 33. 3 0. 26 3. 5 4 (-60-l-80) 2 (48+60) 56. 0 28. 6 0. 206 1 O 5 (60+80) 2 (-48+60) 62. 5 25. 0 0. 17 1 A1 2 (-48-i-60) 2 (48- -6 40. 0 40. 0 0. 59 3. 75 2 (100+12 2 (48 60) 40. 0 40. 0 0. 38 4. 00 0. 5 (60+80) 5. 5 (4860) 15. 0 55. 0 0. 4 2. 25 1 (-60-l-8 50. 0 1. 0 2. 50 A5. 1 (100+120) 4 (-48-1-60) 2 (48+60) 57 28 38 3. 75 A6--- 1 (-100+120) 4 (48+60) 1 (-48-1-60) 67 17 .44 2 A7 1 (100+120) 4 (-48+60) 0.5 (-48+60) 72. 5 4. 5 46 1. 75

Nora-Number in parenthesis is U.S. standard mesh size.

As a specific example of the practice of the invention, the coating compositions as listed in Table I were produced. To determine the adherence properties of the coatings reported in Table I, they were subjected to the following test conditions. The quantity of each of the coating materials was applied to a ten-pound tool steel ingot, which was heated for one hour to a temperature of 1200 F. A single horizontal surface of the heated ingot was coated, and the coated surface was positioned horizontally for five seconds. The coated surface was then turned vertically and the ingot was struck on the adjacent, uncoated surface to disturb the coating. The coated ingot surface was then examined to determine the extent to which the coating was removed during the above-described testing operation. An adherence rating was then given for each of the coating materials tested:

(1) Very poorif to 25 percent of the coating adheres. (2) Poorif 26 to 75 percent of the coating adheres. (3) Fairif 76 to 95 percent of the coating adheres. (4) Goodif 96 to 100 percent of the coating adheres.

In Table I the adherence rating, as well as the adherence ratio for each of the coating materials tested is presented. In addition, these results are plotted in FIG. 1. As may be seen from these data, to achieve an adherence rating of 3 or above, which is necessary for a satisfactory coating, an adherence ratio of at least about 0.2 and preferably of at least about 0.3 is required.

The coatings as listed in Table I were also tested to determine their properties in protecting the coated workpiece surfaces against oxidation and decarburization. In this testing operation, each of the coating materials listed in Table I were applied on four sides of a ten-pound tool steel ingot previously heated to a temperature of 1200 F. for one hour. Each coated ingot was heated for six hours at a temperature of 2100 F. in an atmosphere containing 4 percent excess oxygen. The ingots were then cross sectioned and etched with a solution of nitric acid. The cross sections were evaluated to determine the extent of oxidized scale and decarburized surface layer present. Some of the ingots, to which coatings having varying amounts of alumina had been applied, were evaluated only with respect to the vertical surfaces (as positioned in the furnace) to determine protection of these surfaces as a function of the amount of alumina and thus the viscosity. This data is plotted on FIG. 3. Each of the ingot samples was rated and given an adherence rating as follows:

the standpoint of protection against oxidation and decarburization, it is necessary that the coating have a minimum protection rating of 3, and preferably 3.5 or above. As may be seen from the protection-rating data presented in Table I and plotted in FIG. 2 to provide the desired protection against oxidation and decarburization, the coating must have a silica content within the range of 10 to 55 percent by weight, and preferably within the range of 25 to 45 percent by weight. Again, it must be stressed that although it is necessary to have silica present within this range to provide the necessary protection, it is also essential that in conjunction with a silica content within this range, that the adherence ratio, as described hereinabove, be maintained at least at 0.2 and preferably at least 0.3. Otherwise, the coating will not be continuous and the exposed areas of the workpiece surface will be subjected to oxidation and decarburization to an extent rendering the coating ineffective for its intended use.

Although various embodiments of the invention have been described herein, it is obvious that other adaptations and modifications may be made by those skilled in the art without departing from the scope and spirit of the appended claims.

I claim:

1. A method for protecting a metal workpiece against scaling and decarburization during elevated-temperature processing comprising coating said workpiece, while said workpiece is at an intermediate elevated temperature, with a coating material comprising sodium borate, 10 to 55 weight percent silica and at least 25 weight percent alumina, with the constituents of said coating material being limited to provide an adherence ratio of at least 0.2, thereafter increasing the temperature of said workpiece to permit hot-working thereof and hot-working said workpiece while at said increased temperature.

2. The method of claim 1 wherein said sodium borate is applied to said workpiece, while said workpiece is at an intermediate elevated temperature, to melt said so dium borate onto said workpiece surface, and applying said silica and alumina substantially continuously onto said melted sodium borate to bond the same to said workpiece in unrnelted form.

3. The method of claim 1 wherein said intermediate elevated temperature is above the melting temperature of said sodium borate and said increased hot-working temperature is above the melting temperature of said silica and alumina.

References Cited UNITED STATES PATENTS 3,540,896 11/1970 Flicker 117129 3,178,323 4/1965 Brown et al 1l7-l29 2,785,091 3/1957 Rex 1l7129 3,399,078 8/1968 Bang l17-129 3,032,425 5/ 1962 Leach 106-383 ALFRED L. LEAVITT, Primary Examiner M. F. ESPOSITO, Assistant Examiner U.S. Cl. X.R. 

